Network Working Group L. Iannone
Internet-Draft TU Berlin - Deutsche Telekom
Intended status: Experimental Laboratories AG
Expires: March 16, 2012 D. Saucez
O. Bonaventure
Universite catholique de Louvain
September 13, 2011
LISP Map-Versioningdraft-ietf-lisp-map-versioning-03.txt
Abstract
This document describes the LISP (Locator/ID Separation Protocol)
Map-Versioning mechanism, which provides in-packet information about
Endpoint-ID to Routing Locator (EID-to-RLOC) mappings used to
encapsulate LISP data packets. The proposed approach is based on
associating a version number to EID-to-RLOC mappings and transport
such a version number in the LISP specific header of LISP-
encapsulated packets. LISP Map-Versioning is particularly useful to
inform communicating Ingress Tunnel Routers (ITRs) and Egress Tunnel
Routers (ETRs) about modifications of the mappings used to
encapsulate packets. The mechanism is transparent to legacy
implementations, since in the LISP-specific header and in the Map
Records, bits used for Map-Versioning can be safely ignored by ITRs
and ETRs that do not support the mechanism.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 16, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
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Internet-Draft LISP Map-Versioning September 20111. Introduction
This document describes the Map-Versioning mechanism used to provide
information on changes in the EID-to-RLOC mappings used in the LISP
([I-D.ietf-lisp]) context to perform packet encapsulation. The
mechanism is totally transparent to xTRs not supporting such
functionality. It is not meant to replace any existing LISP
mechanism, but rather to complete them providing new functionalities.
The basic mechanism is to associate a Map-Version number to each LISP
EID-to-RLOC mapping and transport such a version number in the LISP-
specific header. When a mapping changes, a new version number is
assigned to the updated mapping. A change in an EID-to-RLOC mapping
can be a change in the RLOCs set, by adding or removing one or more
RLOCs, but it can also be a change in the priority or weight of one
or more RLOCs.
When Map-Versioning is used, LISP-encapsulated data packets contain
the version number of the two mappings used to select the RLOCs in
the outer header (i.e., both source and destination). These version
numbers are encoded in the 24 low-order bits of the first longword of
the LISP header and indicated by a specific bit in the flags (first 8
high-order bits of the first longword of the LISP header). Note that
not all packets need to carry version numbers.
When an ITR encapsulates a data packet, with a LISP header containing
the Map-Version numbers, it puts in the LISP-specific header two
version numbers:
1. The version number assigned to the mapping (contained in the EID-
to-RLOC Database) used to select the source RLOC.
2. The version number assigned to the mapping (contained in the EID-
to-RLOC Cache) used to select the destination RLOC.
This operation is two-fold. On the one hand, it enables the ETR
receiving the packet to know if the ITR has the latest version number
that any ETR at the destination EID site has provided to the ITR in a
Map-Reply. If it is not the case the ETR can send to the ITR a Map-
Request containing the updated mapping or soliciting a Map-Request
from the ITR (both cases are already defined in [I-D.ietf-lisp]). In
this way the ITR can update its cache. On the other hand, it enables
an ETR receiving such a packet to know if it has in its EID-to-RLOC
Cache the latest mapping for the source EID (in case of bidirectional
traffic). If it is not the case a Map-Request can be sent.
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Internet-Draft LISP Map-Versioning September 20112. Requirements Notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Definitions of Terms
The present document uses terms already defined in main LISP
specification [I-D.ietf-lisp]. Hereafter are defined only the terms
that are specific to the Map-Versioning mechanism.
Map-Version number: An unsigned 12-bits assigned to an EID-to-RLOC
mapping, not including the value 0 (0x000).
Null Map-Version: The 12-bits null value of 0 (0x000) is not used as
Map-Version number. It is used to signal that no Map-Version
number is assigned to the EID-to-RLOC mapping.
Source Map-Version number: Map-Version number of the EID-to-RLOC
mapping used to select the source address (RLOC) of the outer IP
header of LISP-encapsulated packets.
Destination Map-Version number: Map-Version number of the EID-to-
RLOC mapping used to select the destination address (RLOC) of the
outer IP header of LISP-encapsulated packets.
4. EID-to-RLOC Map-Version number
The EID-to-RLOC Map-Version number consists in an unsigned 12-bits
integer. The version number is assigned in a per-mapping fashion,
meaning that different mappings will have assigned a different
version number, which is also updated independently. An update in
the version number (i.e., a newer version) consist in incrementing by
one the older version number. Appendix A contains a rough estimation
of the wrap-around time for the Map-Version number.
The space of version numbers has a circular order where half of the
version numbers is greater than the current Map-Version number and
the other half is smaller than current Map-Version number. In a more
formal way, assuming we have two version numbers V1 and V2 and that
the numbers are expressed on N bits, the following three cases may
happen:
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V1 = V2 : This is the exact match case.
V1 < V2 : True if and only if V1 < V2 < (V1 + 2**(N-1)).
V1 > V2 : True if and only if V1 > V2 > (V1 - 2**(N-1)).
Using 12 bits, as defined in this document, and assuming a Map-
Version value of 69, Map-Version numbers in the range [70; 69 + 2047]
are greater than 69, while Map-Version numbers in the range [69 +
2048; (69 + 4095) mod 4096] are smaller than 69.
The initial Map-Version number of a new EID-to-RLOC mapping SHOULD be
randomly generated. However, it MUST NOT be set to the Null Map-
Version value (0x000), because it has a special meaning (see
Section 4.1).
4.1. The Null Map-Version
The value 0x000 (zero) is not a valid Map-Version number indicating
the version of the EID-to-RLOC mapping. Such a value is used for
special purposes and is named the Null Map-Version number.
The Null Map-Version MAY appear in the LISP specific header as either
Source Map-Version number (cf. Section 5.2) or Destination Map-
Version number (cf. Section 5.1). When the Source Map-Version number
is set to the Null Map-version value it means that no map version
information is conveyed for the source site. This means that if a
mapping exists for the source EID in the EID-to-RLOC Cache, then the
ETR MUST NOT compare the received Null Map-Version with the content
of the EID-to-RLOC cache. When the Destination Map-version number is
set to the Null Map-version value it means that no map version
information is conveyed for the destination site. This means that
the ETR MUST NOT compare the value with the Map-Version number of the
mapping for the destination EID present in the EID-to-RLOC Database.
The other use of the Null Map-Version number is in the Map Records,
which are part of the Map-Request, Map-Reply and Map-Register
messages (defined in [I-D.ietf-lisp]). Map Records that have a Null
Map-Version number indicate that there is no Map-Version number
associated with the mapping. This means that LISP encapsulated
packets, destined to the EID-Prefix the Map Record refers to, MUST
either not contain any Map-Version numbers (V bit set to 0), or if it
contains Map-Version numbers (V bit set to 1) then the destination
Map-Version number MUST be set to the Null Map-Version number. Any
value different from zero means that Map-Versioning is supported and
MAY be used.
The fact that the 0 value has a special meaning for the Map-Version
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number implies that, when updating a Map-Version number because of a
change in the mapping, if the next value is 0 then Map-Version number
MUST be incremented by 2 (i.e., set to 1, which is the next valid
value).
5. Dealing with Map-Version numbers
The main idea of using Map-Version numbers is that whenever there is
a change in the mapping (e.g., adding/removing RLOCs, a change in the
weights due to TE policies, or a change in the priorities) or an ISP
realizes that one or more of its own RLOCs are not reachable anymore
from a local perspective (e.g., through IGP, or policy changes) the
ISP updates the mapping also assigning a new Map-Version number.
To each mapping, a version number is associated and changes each time
the mapping is changed. Note that map-versioning does not introduce
new problems concerning the coordination of different ETRs of a
domain. Indeed, ETRs belonging to the same LISP site must return for
a specific EID-prefix the same mapping, including the same Map-
Version number. In principle this is orthogonal to whether or not
map-versioning is used. The synchronization problem is out of the
scope of this document.
In order to announce in a data-driven fashion that the mapping has
been updated, Map-Version numbers used to create the outer IP header
of the LISP-encapsulated packet are embedded in the LISP-specific
header. This means that the header needs to contain two Map-Version
numbers:
o The Source Map-Version number of the EID-to-RLOC mapping in the
EID-to-RLOC Database used to select the source RLOC.
o The Destination Map-Version number of the EID-to-RLOC mapping in
the EID-to-RLOC Cache used to select the destination RLOC.
By embedding both Source Map-Version number and Destination Map-
Version number an ETR receiving a LISP packet with Map-Version
numbers, can perform the following checks:
1. The ITR that has sent the packet has an up-to-date mapping in its
cache for the destination EID and is performing encapsulation
correctly.
2. In case of bidirectional traffic, the mapping in the local ETR
EID-to-RLOC cache for the source EID is up-to-date.
If one or both of the above conditions do not hold, the ETR can send
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a Map-Request either to make the ITR aware that a new mapping is
available (see Section 5.1) or to update the mapping in the local
cache (see Section 5.2).
5.1. Handling Destination Map-Version number
When an ETR receives a packet, the Destination Map-Version number
relates to the mapping for the destination EID for which the ETR is a
RLOC. This mapping is part of the ETR EID-to-RLOC Database. Since
the ETR is authoritative for the mapping, it has the correct and up-
to-date Destination Map-Version number. A check on this version
number can be done, where the following cases can arise:
1. The packets arrive with the same Destination Map-Version number
stored in the EID-to-RLOC Database. This is the regular case.
The ITR sending the packet has in its EID-to-RLOC Cache an up-to-
date mapping. No further actions are needed.
2. The packet arrives with a Destination Map-Version number greater
(i.e., newer) than the one stored in the EID-to-RLOC Database.
Since the ETR is authoritative on the mapping, this means that
someone is not behaving correctly with respect to the
specifications, thus the packet carries a not valid version
number and SHOULD be silently dropped.
3. The packets arrive with a Destination Map-Version number smaller
(i.e., older) than the one stored in the EID-to-RLOC Database.
This means that the ITR sending the packet has an old mapping in
its EID-to-RLOC Cache containing stale information. The ITR
sending the packet has to be informed that a newer mapping is
available. This is done with a Map-Request message sent back to
the ITR. The Map-Request will either trigger a Map-Request back
using the Solicit-Map-Request (SMR) bit or it will piggyback the
newer mapping. These are not new mechanisms; how to SMR or
piggyback mappings in Map-Request messages is already described
in [I-D.ietf-lisp], while their security is discussed in
[I-D.ietf-lisp-threats]. These Map-Request messages should be
rate limited (rate limitation policies are also described in
[I-D.ietf-lisp]). The feature introduced by Map-Version numbers
is the possibility of blocking traffic not using the latest
mapping. Indeed, after a certain number of retries, if the
Destination Map-Version number in the packets is not updated, the
ETR MAY drop packets with a stale Map-Version number while
strongly reducing the rate of Map-Request messages. This because
either the ITR is refusing to use the mapping for which the ETR
is authoritative or (worse) it might be some form of attack.
Another case might be that the control-plane is experiencing
transient failures so the Map-Requests cannot reach that ITR. By
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keeping sending Map-Requests at very low rate it is possible to
recover from this situation.
The rule in the third case MAY be more restrictive. If the mapping
has been the same for a period of time as long as the TTL (defined in
[I-D.ietf-lisp]) of the previous version of the mapping, all packets
arriving with an old Map-Version SHOULD be silently dropped right
away without issuing any Map-Request. The reason that allows such
action is the fact that if the new mapping with the updated version
number has been unchanged for at least the same time as the TTL of
the older mapping, all the entries in the caches of ITRs must have
expired. Hence, all ITRs sending traffic should have refreshed the
mapping according to [I-D.ietf-lisp]. If packets with old Map-
Version number are still received, then either someone has not
respected the TTL, or it is a form of spoof/attack. In both cases
this is not valid behavior with respect to the specifications and the
packet SHOULD be silently dropped.
LISP-encapsulated packets with the V-bit set, when the original
mapping in the EID-to-RLOC Database has version number set to the
Null Map-Version value, MAY be silently dropped. As explained in
Section 4.1, if an EID-to-RLOC mapping has a Null Map-Version, it
means that ITRs, using the mapping for encapsulation, MUST NOT use
Map-Version number in the LISP-specific header.
For LISP-encapsulated packets with the V-bit set, when the original
mapping in the EID-to-RLOC Database has version number set to a value
different from the Null Map-Version value, a Destination Map-Version
number equal to the Null Map-Version value means that the Destination
Map-Version number MUST be ignored.
5.2. Handling Source Map-Version number
When an ETR receives a packet, the Source Map-Version number relates
to the mapping for the source EID for which the ITR that sent the
packet is authoritative. If the ETR has an entry in its EID-to-RLOC
Cache for the source EID, then a check can be performed and the
following cases can arise:
1. The packet arrives with the same Source Map-Version number stored
in the EID-to-RLOC Cache. This is the correct regular case. The
ITR has in its cache an up-to-date copy of the mapping. No
further actions are needed.
2. The packet arrives with a Source Map-Version number greater
(i.e., newer) than the one stored in the local EID-to-RLOC Cache.
This means that ETR has in its cache a mapping that is stale and
needs to be updated. A Map-Request SHOULD be sent to get the new
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mapping for the source EID. This is a normal Map-Request message
sent through the mapping system and MUST respect the
specifications in [I-D.ietf-lisp], including rate limitation
policies.
3. The packet arrives with a Source Map-Version number smaller
(i.e., older) than the one stored in the local EID-to-RLOC Cache.
Such a case is not valid with respect to the specifications.
Indeed, if the mapping is already present in the EID-to-RLOC
Cache, this means that an explicit Map-Request has been sent and
a Map-Reply has been received from an authoritative source.
Assuming that the mapping system is not corrupted anyhow, the
Map-Version in the EID-to-RLOC Cache is the correct one and the
packet MAY be silently dropped.
If the ETR does not have an entry in the EID-to-RLOC Cache for the
source EID (e.g., in case of unidirectional traffic) then the Source
Map-Version number can be safely ignored.
For LISP-encapsulated packets with the V-bit set, if the Source Map-
Version number is the Null Map-Version value, it means that the
Source Map-Version number MUST be ignored.
6. LISP header and Map-Version numbers
In order for the versioning approach to work, the LISP specific
header has to carry both Source Map-Version number and Destination
Map-Version number. This is done by setting the V-bit in the LISP
specific header as defined in [I-D.ietf-lisp] Section 5.3. When the
V-bit is set the low-order 24-bits of the first longword (which
usually contains the nonce) are used to transport both source and
destination Map-Version numbers. In particular the first 12 bits are
used for Source Map-Version number and the second 12 bits for the
Destination Map-Version number.
Hereafter is the example of LISP header carrying version numbers in
the case of IPv4-in-IPv4 encapsulation. The same setting can be used
for any other case (IPv4-in-IPv6, IPv6-in-IPv4, and IPv6-in-IPv6).
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ |N|L|E|V|I|flags| Source Map-Version |Destination Map-Version|
LISP+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
\ | Instance ID/Locator Status Bits |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Source Map-Version number (12 bits): Map-Version of the mapping used
by the ITR to select the RLOC present in the "Source Routing
Locator" field. How to set on transmission and handle on
reception this value is described in Section 5.2.
Destination Map-Version number (12 bits): Map-Version of the mapping
used by the ITR to select the RLOC present in the "Destination
Routing Locator" field. How to set on transmission and handle on
reception this value is described in Section 5.1.
The present document just specifies how to use the low-order 24-bits
of the first longword of the LISP-specific header when the V-bit is
set to 1. All other cases, including the bit fields of the rest of
the LISP-specific header and the whole LISP packet format are
specified in [I-D.ietf-lisp]. Not all of the LISP encapsulated
packets need to carry version numbers. When Map-Version numbers are
carried the V-bit MUST be set to 1. All legal combinations of the
flags, when the V-bit is set to 1, are described in [I-D.ietf-lisp].
7. Map Record and Map-Version
To accommodate the proposed mechanism, the Map Records that are
transported on Map-Request/Map-Reply/Map-Register messages need to
carry the Map-Version number as well. For this purpose the 12-bits
before the EID-AFI field in the Record that describe a mapping is
used. This is defined in [I-D.ietf-lisp] and reported here as
example.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Record TTL |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R | Locator Count | EID mask-len | ACT |A| Reserved |
e +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
c | Rsvd | Map-Version Number | EID-AFI |
o +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
r | EID-prefix |
d +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /| Priority | Weight | M Priority | M Weight |
| L +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| o | Unused Flags |L|p|R| Loc-AFI |
| c +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| \| Locator |
+-> +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Map-Version Number: Map-Version of the mapping contained in the
Record. As explained in Section 4.1 this field can be zero (0),
meaning that no Map-Version is associated to the mapping, hence
packets that are LISP-encapsulated using this mapping MUST NOT
contain Map-Version numbers in the LISP specific header and the
V-bit MUST be set to 0.
This packet format works perfectly with xTRs that do not support Map-
Versioning, since they can simply ignore those bits.
8. Benefits and case studies for Map-Versioning
In the following sections we provide more discussion on various
aspects and use of the Map-Versioning. Security observations are
instead grouped in Section 10.
8.1. Synchronization of different xTRs
Map-Versioning does not require additional synchronization mechanism
compared to the normal functioning of LISP without Map-Versioning.
Clearly all the ETRs have to reply with the same Map-Version number,
otherwise there can be an inconsistency that creates additional
control traffic, instabilities, traffic disruptions. It is the same
without Map-Versioning, with ETRs that have to reply with the same
mapping, otherwise the same problems can arise.
As an example, let's consider the topology of Figure 1 where ITR A.1
of domain A is sending unidirectional traffic to the domain B, while
A.2 of domain A exchange bidirectional traffic with domain B. In
particular, ITR A.2 send traffic to ETR B and ETR A.2 receives
traffic from ITR B.
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+-----------------+ +-----------------+
| Domain A | | Domain B |
| +---------+ | |
| | ITR A.1 |--- | |
| +---------+ \ +---------+ |
| | ------->| ETR B | |
| | ------->| | |
| +---------+ / | | |
| | ITR A.2 |--- -----| ITR B | |
| | | / +---------+ |
| | ETR A.2 |<----- | |
| +---------+ | |
| | | |
+-----------------+ +-----------------+
Figure 1
Obviously in the case of Map-Versioning both ITR A.1 and ITR A.2 of
domain A must use the same value otherwise the ETR of domain B will
start to send Map-Requests.
The same problem can, however, arise without Map-Versioning. For
instance, if the two ITRs of domain A send different Loc Status Bits.
In this case either the traffic is disrupted, if the ETR B trusts the
Locator Status Bits, or if ETR B does not trusts the Locator Status
Bits it will start sending Map-Requests to confirm the each change in
the reachability.
So far, LISP does not provide any specific synchronization mechanism,
but assumes that synchronization is provided by configuring the
different xTRs consistently. The same applies for Map-Versioning.
If in the future any synchronization mechanism is provided, Map-
Versioning will take advantage of it automatically since it is
included in the Record format, as described in Section 7.
8.2. Map-Versioning and unidirectional traffic
When using Map-Versioning the LISP specific header carries two Map-
Version numbers, for both source and destination mappings. This can
raise the question on what will happen in the case of unidirectional
flows, like for instance in the case presented in Figure 2, since
LISP specification do not mandate for ETR to have a mapping for the
source EID.
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+-----------------+ +-----------------+
| Domain A | | Domain B |
| +---------+ +---------+ |
| | ITR A |----------->| ETR B | |
| +---------+ +---------+ |
| | | |
+-----------------+ +-----------------+
Figure 2
For what concerns the ITR, it is able to put both source and
destination version number in the LISP header since the Source Map-
Version number is in ITR's database, while the Destination Map-
Version number is in ITR's cache.
For what concerns the ETR, it simply checks only the Destination Map-
Version number in the same way as described in Section 5, ignoring
the Source Map-Version number.
8.3. Map-Versioning and interworking
Map-Versioning is compatible with the LISP interworking between LISP
and non-LISP sites as defined in [I-D.ietf-lisp-interworking]. LISP
interworking defines three techniques to make LISP sites and non-LISP
sites, namely Proxy-ITR, LISP-NAT, and Proxy-ETR. Hereafter it is
described how Map-Versioning relates to these three mechanisms.
8.3.1. Map-Versioning and Proxy-ITRs
The purpose of the Proxy-ITR (PITR) is to encapsulate traffic
originating in a non-LISP site in order to deliver the packet to one
of the ETRs of the LISP site (cf. Figure 3). This case is very
similar to the unidirectional traffic case described in Section 8.2,
hence similar rules apply.
+----------+ +-------------+
| LISP | | non-LISP |
| Domain A | | Domain B |
| +-------+ +-----------+ | |
| | ETR A |<-------| Proxy ITR |<-------| |
| +-------+ +-----------+ | |
| | | |
+----------+ +-------------+
Figure 3
The main difference is that a Proxy-ITR does not have any mapping,
since it just encapsulate packets arriving from non-LISP site, thus
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cannot provide a Source Map-Version. In this case, the proxy-ITR
will just put the Null Map-Version value as Source Map-Version
number, while the receiving ETR will ignore the field.
With this setup the LISP Domain A is able to check whether or not the
PITR is using the latest mapping. If this is not the case the
mapping for LISP Domain A on the PITR can be updated using one of the
mechanisms defined in [I-D.ietf-lisp] and
[I-D.ietf-lisp-interworking].
8.3.2. Map-Versioning and LISP-NAT
The LISP-NAT mechanism is based on address translation from non-
routable EIDs to routable EIDs and does not involve any form of
encapsulation. As such Map-Versioning does not apply in this case.
8.3.3. Map-Versioning and Proxy-ETRs
The purpose of the Proxy-ETR (PETR) is to decapsulate traffic
originating in a LISP site in order to deliver the packet to the non-
LISP site (cf. Figure 4). One of the main reasons of deploy PETRs is
to bypass uRPF (Unicast Reverse Path Forwarding) checks on the
provider edge.
+----------+ +-------------+
| LISP | | non-LISP |
| Domain A | | Domain B |
| +-------+ +-----------+ | |
| | ITR A |------->| Proxy ETR |------->| |
| +-------+ +-----------+ | |
| | | |
+----------+ +-------------+
Figure 4
A Proxy-ETR does not have any mapping, since it just decapsulates
packets arriving from LISP site. In this case, the ITR will just put
the Null Map-Version value as Destination Map-Version number, while
the receiving Proxy-ETR will ignore the field.
With this setup the Proxy-ETR is able to check whether or not the
mapping has changed. If this is the case the mapping for LISP Domain
A on the PETR can be updated using one of the mechanisms defined in
[I-D.ietf-lisp] and [I-D.ietf-lisp-interworking].
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Internet-Draft LISP Map-Versioning September 20118.4. RLOC shutdown/withdraw
Map-Versioning can be even used to perform a graceful shutdown or
withdraw of a specific RLOC. This is achieved by simply issuing a
new mapping, with an updated Map-Version number, where the specific
RLOC to be shut down is withdrawn or announced as unreachable (R bit
in the Map Record, see [I-D.ietf-lisp]), but without actually turning
it off.
Once no more traffic is received by the RLOC, it can be shut down
gracefully, because at least all sites actively using the mapping
have updated it.
It should be pointed out that for frequent up/down changes such a
mechanism should not be used since this can generate excessive load
on the Mapping System.
8.5. Map-Version for lightweight LISP implementation
The use of Map-Versioning can help in developing a lightweight
implementation of LISP. This comes with the price of not supporting
Loc-Status-Bit, which are useful in some contexts.
In the current LISP specifications the set of RLOCs must always be
maintained ordered and consistent with the content of the Loc Status
Bits (see section 6.5 of [I-D.ietf-lisp]). With Map-Versioning such
type of mechanisms can be avoided. When a new RLOC is added to a
mapping, it is not necessary to "append" new locators to the existing
ones as explained in Section 6.5 of [I-D.ietf-lisp]. A new mapping
with a new Map-Version number will be issued, and since the old
locators are still valid the transition will be with no disruptions.
The same applies for the case a RLOC is withdrawn. There is no need
to maintain holes in the list of locators, as is the case when using
Locator Status Bits, for sites that are not using the RLOC that has
been withdrawn the transition will be with no disruptions.
All of these operations, as already stated, do not need to maintain
any consistency among Locator Status Bits, and the way RLOC are
stored in the cache.
Further, Map-Version can be used to substitute the "clock sweep"
operation described in Section 6.5.1 of [I-D.ietf-lisp]. Indeed,
every LISP site communicating to a specific LISP site that has
updated the mapping will be informed of the available new mapping in
a data-driven manner.
Note that what is proposed in the present section is just an example
and MUST NOT be considered as specifications for a lightweight LISP
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implementation. In case the IETF decides to undertake such a work,
it will be documented elsewhere.
9. Incremental deployment and implementation status
Map-Versioning can be incrementally deployed without any negative
impact on existing LISP elements (e.g., xTRs, Map-Servers, Proxy-
ITRs, etc). Any LISP element that does not support Map-Versioning
can safely ignore them. Further, there is no need of any specific
mechanism to discover if an xTR supports or not Map-Versioning. This
information is already included in the Map Record.
Map-Versioning is currently implemented in OpenLISP
[I-D.iannone-openlisp-implementation].
Note that the reference document for LISP implementation and
interoperability tests remains [I-D.ietf-lisp].
10. Security Considerations
Map-Versioning does not introduce any security issue concerning both
the data-plane and the control-plane. On the contrary, as described
in the following, if Map-Versioning may be used also to update
mappings in case of change in the reachability information (i.e.,
instead of the Locator Status Bits) it is possible to reduce the
effects of some DoS or spoofing attacks that can happen in an
untrusted environment.
Robusteness of the Map-Versioning mechanism leverages on a trusted
Mapping Distribution System. A thorough security analysis of LISP is
documented in [I-D.ietf-lisp-threats].
10.1. Map-Versioning against traffic disruption
An attacker can try to disrupt ongoing communications by creating
LISP encapsulated packets with wrong Locator Status Bits. If the xTR
blindly trusts the Locator Status Bits it will change the
encapsulation accordingly, which can result in traffic disruption.
This does not happen in the case of Map-Versioning. As described in
Section 5, upon a version number change the xTR first issues a Map-
Request. The assumption is that the mapping distribution system is
sufficiently secure that Map-Request and Map-Reply messages and their
content can be trusted. Security issues concerning specific mapping
distribution system are out of the scope of this document. In the
case of Map-Versioning the attacker should "guess" a valid version
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number that triggers a Map-Request, as described in Section 5,
otherwise the packet is simply dropped. Nevertheless, guessing a
version number that generates a Map-Request is easy, hence it is
important to follow the rate limitations policies described in
[I-D.ietf-lisp] in order to avoid DoS attacks.
Note that a similar level of security can be obtained with Loc Status
Bits, by simply making mandatory to verify any change through a Map-
Request. However, in this case Locator Status Bits loose their
meaning, because, it does not matter anymore which specific bits has
changed, the xTR will query the mapping system and trust the content
of the received Map-Reply. Furthermore there is no way to perform
filtering as in the Map-Versioning in order to drop packets that do
not carry a valid Map-Version number. In the case of Locator Status
Bits, any random change can trigger a Map-Request (unless rate
limitation is enabled which raise another type of attack discussed in
Section 10.2).
10.2. Map-Versioning against reachability information DoS
Attackers can try to trigger a large amount of Map-Request by simply
forging packets with random Map-Version or random Locator Status
Bits. In both cases the Map-Requests are rate limited as described
in [I-D.ietf-lisp]. However, differently from Locator Status Bit
where there is no filtering possible, in the case of Map-Versioning
is possible to filter not valid version numbers before triggering a
Map-Request, thus helping in reducing the effects of DoS attacks. In
other words the use of Map-Versioning enables a fine control on when
to update a mapping or when to notify that a mapping has been
updated.
It is clear, that Map-Versioning does not protect against DoS and
DDoS attacks, where an xTR looses processing power doing checks on
the LISP header of packets sent by attackers. This is independent
from Map-Versioning and is the same for Loc Status Bits.
11. IANA Considerations
This document has no actions for IANA.
12. Acknowledgements
The authors would like to thank Alia Atlas, Jesper Skriver, Pierre
Francois, Noel Chiappa, Dino Farinacci for their comments and review.
This work has been partially supported by the INFSO-ICT-216372
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granularity of seconds and assuming as worst-case that a new version
is issued each second, it takes slightly more than 1 hour before the
version wraps around. Note that the granularity of seconds is in
line with the rate limitation policy for Map-Request messages, as
proposed in the LISP main specifications ([I-D.ietf-lisp]).
Alternatively a granularity of minutes can also be used, as for the
TTL of the Map-Reply ([I-D.ietf-lisp]). In this case the worst
scenario is when a new version is issued every minute, leading to a
much longer time before wrap-around. In particular, when using 12
bits, the wrap-around time is almost 3 days.
For general information, hereafter there is a table with a rough
estimation of the time before wrap-around in the worst-case scenario,
considering different sizes (bits length) of the Map-Version number
and different time granularity.
+---------------+--------------------------------------------+
|Version Number | Time before wrap around |
| Size (bits) +---------------------+----------------------+
| |Granularity: Minutes | Granularity: Seconds |
| | (mapping changes | (mapping changes |
| | every 1 minute) | every 1 second) |
+-------------------------------------+----------------------+
| 32 | 8171 Years | 136 Years |
| 30 | 2042 Years | 34 Years |
| 24 | 31 Years | 194 Days |
| 16 | 45 Days | 18 Hours |
| 15 | 22 Days | 9 Hours |
| 14 | 11 Days | 4 Hours |
| 13 | 5.6 Days | 2.2 Hours |
| 12 | 2.8 Days | 1.1 Hours |
+---------------+---------------------+----------------------+
Figure 5: Estimation of time before wrap-around
Appendix B. Document Change Log
o Version 03 Posted September 2011.
* Added reference in Section 7 toward the main lisp documents
specifying the section, as requested by Jari Arkko.
* Fixed all typos and editorial issues pointed out by Jari Arkko.
* Added clarification in Section 8.4 as requested by Jari Arkko.
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* Extentend all acronyms in the abstract as requested by Jari
Arkko.
* Clarified silent drop polocy in Section 5.2 as requested by
both Richard Barnes and Jari Arkko.
* Fixed typos pointed out by Richard Barnes.
o Version 02 Posted July 2011.
* Added text in Section 5 about ETR synchronization, as suggested
by Alia Atlas.
* Modified text in Section 8.5 concerning lightweight LISP
implementation, as suggested by Alia Atlas.
* Deleted text concerning old versions of [I-D.ietf-lisp-ms] and
[I-D.ietf-lisp-alt] in Section 7, as pointed out by Alia Atlas.
* Fixed section 4.1 to be less restrictive, as suggested by
Jesper Skriver.
o Version 01 Posted March 2011.
* Changed the wording from "Map-Version number 0" to "Null Map-
Version.
* Clarification of the use of the Null Map-Version value as
Source Map-Version Number and Destination Map-Version Number.
* Extended the section describing Map-Versioning and LISP
Interworking co-existence.
* Reduce packet format description to avoid double definitions
with the main specs.
o Version 00 Posted September 2010.
* Added Section "Definitions of Terms".
* Editorial polishing of all sections.
* Added clarifications in section "Dealing with Map-Version
numbers" for the case of the special Map-Version number 0.
* Rename of draft-iannone-mapping-versioning-02.txt.
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